For almost 45 years, Royal Navy submariners have provided round-the-clock at-sea deterrence, delivering the ultimate guarantee of our national security. Carrying first Polaris and now Trident ballistic missiles, the undetected patrolling of our nuclear-armed submarines provides an essential protection against nuclear blackmail or attack.

Although no state currently has both the intent and the capability to threaten the integrity of the UK, there are countries in unstable regions that possess, or are on the verge of possessing, nuclear weapons. North Korea has tested nuclear devices and ballistic missiles. Iran appears bent on the production of highly enriched uranium and continues to develop a ballistic missile capability. And no one can predict the nature of the threats we will face in the future. In the face of this uncertainty, it would be deeply irresponsible for any British government to dismiss the possibility of a nuclear threat to our country.

That is why, in May 2010, Conservatives and Liberal Democrats confirmed in the Coalition’s Programme for Government that “we will maintain Britain’s nuclear deterrent”. Of course, there are differences between the two parties about how deterrence should be delivered. So, while the 2010 Strategic Defence and Security Review announced that we would proceed with replacing the Vanguard submarines as they come to the end of their life in the late 2020s, the Programme for Government specified that the “Liberal Democrats will continue to make the case for alternatives”. The current Trident Alternatives Review was designed to help them do that.

Yet despite our differences on the means, we were clear on the nature of the deterrent we require: to be credible, it must provide effective deterrence against the full range of current and future threats. With the Vanguard Successor submarines costing some £11-14 billion at 2006/07 prices and delivering security to 2060 and beyond, the deterrent must be carefully scrutinised for savings. If there is a more cost-effective way of delivering the required deterrence, of course we should investigate it, and we have already identified more than £1 billion of cost reductions. But we must be under no illusion that we can pick and choose the threats we face. A deterrent only deters if it is credible and available. All the evidence points to a continuous at-sea presence, based on Trident, as the most cost-effective route to deliver the deterrent effect.

Some have argued that we should sacrifice our continuous at-sea deterrence. But not having a submarine permanently at sea would make us vulnerable to a pre-emptive strike. What is more, having to take the decision to arm and deploy our deterrent at sea in a period of tension would risk escalation at the critical moment. And although it may seem counter-intuitive, the evidence points to a replacement for Vanguard being a lower-cost solution than the proposal for a less capable option based on Astute submarines with nuclear-tipped cruise missiles. The Vanguard replacement would use existing warheads and missiles and elements of the submarine would be designed in collaboration with the US. The cruise alternative would mean designing new warheads and missiles, without American partnership, as well as making major modifications to the launch submarines – and the greater vulnerability of cruise missiles means we would need many more of them to deliver any meaningful effect. A cruise-based deterrent would carry significant risk of miscalculation and unintended escalation. At the point of firing, other states could have no way of knowing whether we had launched a conventional cruise missile or one with a nuclear warhead. Such uncertainty could risk triggering a nuclear war at a time of tension. So, the cruise option would carry enormous financial, technical and strategic risk.

Trident remains the best option for Britain. By keeping our deterrent at sea, we maximise our freedom of manoeuvre, while the Trident missile offers range, endurance and invulnerability with the cost savings of operating a common system with the US. At around 5 to 6 per cent of the annual defence budget, the deterrent is affordable and reinforces our special relationship with the US and our status as a force for good in the world.